The UK’s Serious Fraud Office has closed an extensive investigation into the foreign exchange market, saying there is “insufficient evidence for a realistic prospect of conviction”.

City of London

The UK’s top antifraud agency said in a statement on March 15 it had uncovered “reasonable grounds to suspect the commission of offences involving serious or complex fraud”, but that the evidence would not be enough to mount a prosecution.

The SFO’s investigation lasted more than 18 months and involved more than half a million documents, the agency said. The SFO said that it is continuing to liaise with the US Department of Justice over its ongoing investigation.

The SFO launched its investigation into the global currencies market in 2014, following a referral from the UK’s Financial Conduct Authority. While the FCA investigates financial crime, the SFO focuses on investigating and prosecuting serious and complex fraud.

A wider international investigation into the manipulation of currency markets involving global regulators including the UK’s FCA and US’s Department of Justice began in 2013, and has resulted in some of the world’s largest foreign exchange dealing banks being hit with more than $10 billion in fines by US and UK authorities.

The SFO scored a high-profile success when former bank trader Tom Hayes was convicted of trying to fraudulently rig the London interbank offered rate, or Libor. However the organisation was dealt a blow in January when six former brokers were acquitted of conspiring with Hayes to rig the benchmark rate.

• This article appeared on WSJ City, a made-for-mobile app that combines the best of The Wall Street Journal and Financial News, tailored for an audience in the City of London. Download here